When turning many low-carbon steels, medium-carbon steels or low-alloy steels with coated cemented carbide inserts, Al.sub.2 O.sub.3 is not the best coating material. The present author has studied the behavior of different coating materials, for example in cutting ferritic pearlitic steels (S. Ruppi, Internal Report) and martensitic quenched and tempered steels both with and without Ca-treatment, (S. Ruppi et al., "Wear Characteristics of TiC, TiCN, TiN and Al.sub.2 O.sub.3 Coatings in the Turning of Conventional and Ca-Treated Steels", International Journal of Refractory Metals & Hard Materials, to be published). In the cutting of these workpiece materials with cemented carbide inserts with various coatings, the Al.sub.2 O.sub.3 layers could be characterized as being the worst coating material. It has also been noticed that .alpha.-Al.sub.2 O.sub.3 does not exhibit better wear properties than .kappa.-Al.sub.2 O.sub.3 in steel, although .alpha.-Al.sub.2 O.sub.3 is better in cast iron. It should, however, be noted that Al.sub.2 O.sub.3 was used among the best coating materials together with TiN as far as notch wear was concerned. In use, .kappa.-Al.sub.2 O.sub.3 is harder on the flank face (where it does not transform into .alpha.-Al.sub.2 O.sub.3). On the rake face, it will transform relatively fast into .alpha.-Al.sub.2 O.sub.3, thus exhibiting the same properties as .alpha.-Al.sub.2 O.sub.3 on the rake face. Also, .kappa.-Al.sub.2 O.sub.3 has a lower conductivity than .alpha.-Al.sub.2 O.sub.3. In fact, the thermal conductivity of .kappa.-Al.sub.2 O.sub.3 is 1/3 of that of .alpha.-Al.sub.2 O.sub.3 (D. G. Gahill et al., "Thermal Conductivity of .kappa.-Al.sub.2 O.sub.3 and .alpha.-Al.sub.2 O.sub.3 Wear Resistant Coatings", Journal of Applied Physics, vol. 83, no. 11, 1 June 1998). This means that the .kappa.-Al.sub.2 O.sub.3 phase can be applied as an effective thermal barrier and should in this respect, be preferred to .alpha.-Al.sub.2 O.sub.3. This is important in steel cutting where high temperatures are encountered and, in general, in those applications where it is important to reduce the temperature flow into the substrate. Consequently, the plastic deformation of the substrate can be reduced. The Al.sub.2 O.sub.3 layer has to be protected from wear in steel, i.e., a relatively thick coating of, for example, TiCN, which has been found to be the best coating material in steel, has to be deposited atop it. Further, the coating on .kappa.-Al.sub.2 O.sub.3 must be deposited at a relatively low temperature than that of conventional CVD in order to avoid the phase transformation of the metastable .kappa.-Al.sub.2 O.sub.3 into .alpha.-Al.sub.2 O.sub.3. It is well-known that the .kappa. to .alpha. transformation is very temperature sensitive. See, for example, FIG. 5 in S. Vuorinen et al., "Phase Transfornation in Chemically Vapour Deposited .kappa.-Al.sub.2 O.sub.3 ", Thin Solid Films, 214(1992) pp. 132-143.
In U.S. Pat. No. 5,137,774, the increased performance of .alpha.-Al.sub.2 O.sub.3 as compared to .kappa.-Al.sub.2 O.sub.3 as a coating on a cemented carbide insert when turning cast iron was shown. In addition, in U.S. Pat. Nos. 5,635,247 and 5,700,569 and 6,015,614, various Al.sub.2 O.sub.3 -coated cemented carbide inserts in which the Al.sub.2 O.sub.3 is deposited on a Ti(C,N) layer or multilayers are shown. However, in tests as conducted by the present inventor, it was noted that the adhesion of an .alpha.-Al.sub.2 O.sub.3 layer to the underlying TiCN layer as well as the adhesion of the TiCN layer to the cemented carbide substrate was often unsatisfactory when the insert was used in the turning of cast iron. The coating failed due to edge chipping which resulted in accelerated wear.
The main reasons for edge chipping have been identified by the present inventor from these tests to be the weak substrate-coating adhesion as well as the weak bond between TiCN and .alpha.-Al.sub.2 O.sub.3.
In one study of the TiC-cemented carbide interface of a 6 .mu.m thick CVD-deposited TiC layer by Vuorinen et al., "TEM Study of Microstructure and Crystallography at the TiC/Cemented Carbide Interface", Science of Hard Materials, 1983, pp. 433-447, it was found by transmission electron microscopy (TEM) that the TiC layer is composed of two regions. Close to the substrate and extending to a thickness of 1.5-2 .mu.m is a layer of fine, equiaxed TiC grains. Above that is a layer of larger (typically 2-4 .mu.m) grains of TiC.
In another study published in Thin Solid Films, 232 (1993) pp. 73-82, Vuorinen et al., entitled "Interfacial Characterization of Chemically Vapour Deposited Titanium Carbide on Cemented Carbide", TiC coatings were CVD-deposited on cemented carbide substrates under non-carburizing conditions. In the absence of .eta.-carbide, it was found that the TiC nucleated and grew epitaxially on both {0001}- and {1010}-WC planes.
The search is continued for improved coatings for coated cemented carbide inserts for cutting steel bodies.